The alkaline hydrolysis of polyacrylonitrile is well known, it was, however, usually carried out in heterogeneous systems using as starting material undissolved, either non-swelled or only slightly swelled polyacrylonitrile in bulk--e.g. sheets--or in the form of a powder. The alkaline hydrolysis of polyacrylonitrile takes place at a comparatively high rate so that solid polymer is hydrolyzed gradually, layer by layer. If the alkaline hydrolysis or saponification was performed thoroughly, the product consisted almost entirely of the corresponding salt of polyacrylic acid. More often, however, the product was a varied mixture of copolymers of acrylic acid with acrylonitrile, containing moreover various cyclic and crosslinked components, particularly if the starting polymer was high-molecular. Chemical composition ranged from almost intact polyacrylonitrile to almost pure polyacrylic acid, depending also on the molecular weight, the size of the particles of the starting polymer etc. Such heterogeneous hydrolysis never resulted in homogeneous gels of the above defined structure. As the hydrolyzates contained usually units of acrylic acid in a predominating amount, it was often supposed that the alkaline hydrolysis is a pure saponification where nitrile groups are immediately transformed to carboxylic ones, the step characteristic for acid hydrolysis leading to amide group being avoided. In the practice, only low-molecular polyacrylonitrile was used for the alkaline hydrolysis, yielding water-soluble products which are utilized as textile sizes, antistatic preparations in the manufacture of synthetic fibres etc.
On the other hand it is known that homogeneous acid hydrolysis of polyacrylonitrile or its copolymers with at least 80% of acrylonitrile units can be controlled in such a way that a multi-block copolymer with high strength and elasticity is formed as a result of the prevailing "zipper-like" step of the hydrolysis. Said multi-block copolymers contained, in addition to intact polyacrylonitrile sequences, comparatively long sequences consisting almost entirely of acrylamide units. Only at high conversions, and particularly at high temperatures of the hydrolysis, the hydrophilic segments contain also some units of acrylic acid and diacryl imide. It has been suggested to increase the portion of acid units therein by additional alkaline hydrolysis or by a treatment with nitrous acid, but most of the amide groups remained intact due to steric hindrances and the end products were different both qualitatively and quantitatively from the hydrogels of the present invention.